Global analytical potential energy surface for the electronic ground state of NH₃ from high level ab initio calculations

The analytical, full-dimensional, and global representation of the potential energy surface of NH₃ in the lowest adiabatic electronic state developed previously (Marquardt, R.; et al. J. Phys. Chem. B 2005, 109, 8439-8451) is improved by adjustment of parameters to an enlarged set of electronic energies from ab initio calculations using the coupled cluster method with single and double substitutions and a perturbative treatment of connected triple excitations (CCSD(T)) and the method of multireference configuration interaction (MRCI). CCSD(T) data were obtained from an extrapolation of aug-cc-pVXZ results to the basis set limit (CBS), as described in a previous work (Yurchenko, S.N.; et al. J. Chem. Phys 2005, 123, 134308); they cover the region around the NH₃ equilibrium structures up to 20 000 hc cm ^-1. MRCI energies were computed using the aug-cc-pVQZ basis to describe both low lying singlet dissociation channels. Adjustment was performed simultaneously to energies obtained from the different ab initio methods using a merging strategy that includes 10 000 geometries at the CCSD(T) level and 500 geometries at the MRCI level. Characteristic features of this improved representation are NH₃ equilibrium geometry r^eq(NH ₃) ≈ 101.28 pm, α^eq(NH₃) ≈ 107.03, the inversion barrier at r^inv(NH₃) ≈ 99.88 pm and 1774 hc cm^-1 above the NH₃ minimum, and dissociation channel energies 41 051 hc cm^-1 (for NH₃ → ( ²B₂)NH₂ + (²S_1/2)H) and 38 450 hc cm^-1 (for NH₃ → (³Σ ^-)NH +(¹Σg⁺)H₂); the average agreement between calculated and experimental vibrational line positions is 11 cm^-1 for ¹⁴N¹H₃ in the spectral region up to 5000 cm^-1. A survey of our current knowledge on the vibrational spectroscopy of ammonia and its isotopomers is also given.